Process for production of a paper or paperboard product

ABSTRACT

The present invention relates to a process for producing a paper or paperboard product which process comprises the steps of, providing a furnish comprising fibers, adding starch to the furnish, adding microfibrillated cellulose to the furnish, conducting the furnish to a wire in order to form a web, wherein the starch and microfibrillated cellulose is added separately to the furnish.

FIELD OF THE INVENTION

The present invention relates to a process for production of a paper or paperboard product comprising a furnish which comprises starch and microfibrillated cellulose.

BACKGROUND

In papermaking processes there is an ongoing concern to find ways to produce paper or paperboard at reduced costs without impairing the properties of the product, e.g. without decreasing the strength of the product.

One way to reduce the cost is to increase the filler content of a paper or paperboard product and thus be able to reduce the amount of fibers in the paper or paperboard. Besides being economically beneficial, fillers also improve the opacity and printability properties of the product. However, large amount of fillers in the product decreases the strength. Thus, there is a balance between the possible amount of fillers added and the required strength of the paper or paperboard produced.

Furthermore, during production of paperboard there is a desire to produce a strong but yet low density product. When increasing the strength of a paperboard the density normally increases. There is thus a balance between the desired strength and the density of the paperboard product.

It is possible to compensate for the decrease in strength, caused for example by addition of large amount of filler or by increased bulk, by improving the fiber bonding properties between the fibers in the paper or paperboard, thereby maintaining the strength. The predominant treatment for improving paper or paperboard strength, particularly dry strength, has so far been to add a strength agent, preferably cationic starch, to the furnish prior to the sheet forming operation. Cationic starch molecules added to the furnish can adhere to the naturally anionic pulp fibers by electrostatic attraction and thus be retained in the wet fiber mat and remain in the final paper or paperboard.

When adding large amounts of cationic starch to a papermaking furnish, in order to achieve high resulting paper strength, two major problems arise. The first is that the cationic starch molecules tend to saturate the anionic charge on the cellulose fibers, thus setting a limit to the amount of cationic starch which can be added to the slurry. If an excess of cationic starch is added, only a portion of the starch added will be retained in the sheet, and the rest will circulate in the paper or board machine white water system. A second problem is that fibers which are made cationic by excessive cationic starch addition will not be able to adsorb other cationic additives which are commonly added to the pulp slurry, such as sizing agents and retention aids. Furthermore, high amounts of starch often cause problems with runnability, microbiology and foaming during the production process.

It has recently been found that addition of microfibrillated cellulose to a paper or board will increase the strength of the product, probably due to the improved fiber bonding.

However, there is still a need for a cost efficient product with good strength properties.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process which in an easy and cost efficient way will be able to produce a paper or paperboard with improved strength and density.

These objects and other advantages are achieved by the paper or paperboard product according to claim 1. The present invention relates to a process for producing a paper or paperboard product which process comprises the steps of; providing a furnish comprising fibers, adding starch to the furnish, adding microfibrillated cellulose to the furnish and conducting the furnish to a wire in order to form a web, wherein the starch and microfibrillated cellulose is added separately to the furnish. It has been shown that a product comprising both starch and microfibrillated cellulose (MFC) increases the strength of the product since the amount of starch can be increased without increasing the density of the paper or paperboard product.

It is preferred that the furnish comprises 2-15% by weight of starch and 1-15% by weight of microfibrillated cellulose. The amount of starch respectively MFC of the product depends on the end use and the corresponded desired properties of the product. High amounts of starch will increase the strength of the product and it has been shown that the combination of MFC and starch makes it possible for the product to retain larger amounts of starch.

The web being formed by said furnish comprising starch and microfibrillated cellulose preferably forms a layer of the paper or paperboard product. The paper or paperboard product is preferably a multilayer product which comprises at least two layers. It may be preferred that the product comprises at least three layers and that the layer located in the middle of the product comprises furnish comprising starch and microfibrillated cellulose, i.e. the web being formed by the furnish that comprises starch and MFC, forms a middle layer of the paper or paperboard.

DETAILED DESCRIPTION

The invention relates to a process for production of a paper or paperboard product comprising a furnish which comprises starch and microfibrillated cellulose. The furnish comprises starch in an amount of 2-15% by weight, preferably between 3-5% by weight. Consequently, the product thus comprises starch of an amount of 2-15% by weight, preferably between 3-5% by weight.

The furnish preferably comprises cellulosic fibers. The cellulosic fibers may be hardwood and/or softwood fibers. The cellulosic fibers may be mechanically, chemimechanically and/or chemically treated. The fibers may also be bleached or unbleached.

It has been found that the addition of starch and microfibrillated cellulose (MFC) to a furnish has a very good effect on the strength of the paper or paperboard. Products comprising both starch in large amounts and MFC, makes it possible for the product to retain larger amount of starch. Surprisingly, it has also been found that the addition of both starch, in large amounts, and MFC do not increase the densification of the product. The combination of MFC and starch thus has a synergistic effect when it comes to boosting the strength of a paper or paperboard product without increasing the density of the product to the same extend. It is thus possible to decrease the density of the paper or paperboard product but still be able to produce a strong product. In this way a much cheaper product can be produced since the fiber content of the product can be decreased and the strength can be remained (compared to products produced according to prior art). Furthermore, the weight of the product will be decreased which will reduce costs during transportation and handling.

Microfibrillated cellulose (MFC) (also known as nanocellulose) is a material made from wood cellulose fibers, where the individual microfibrils have been partly or totally detached from each other. MFC is normally very thin (˜20 nm) and the length is often between 100 nm to 10 μm. However, the microfibrils may also be longer, for example between 10-100 μm but lengths up to 200 μm can also be used. Fibers that has been fibrillated and which have microfibrils on the surface and microfibrils that are separated and located in a water phase of a slurry are included in the definition MFC.

MFC can be produced in a number of different ways. It is possible to mechanically treat cellulosic fibers so that microfibrils are formed. The production of nanocellulose or microfibrillated cellulose with bacteria is another option. It is also possible to produce microfibrils from cellulose by the aid of different chemicals and/or enzymes which will break or dissolve the fibers.

One example of production of MFC is shown in WO2007091942 which describes production of MFC by the aid of refining in combination with addition of an enzyme.

It is also possible to modify the microfibrillated cellulose before addition to the furnish. In this way it is possible to change its interaction and affinity to other substances. For example, by introducing more anionic charges to MFC the stability of the fibril and fibril aggregates of the MFC are increased. How the modification of the microfibrillated fibers is done depends, for example on the other components present in the furnish.

The furnish comprises 2-15% by weight of starch, preferably between 3-5% by weight, and it is preferred that the furnish further comprises MFC in an amount of 1-15% by weight. The chosen amount of starch respectively MFC added to the furnish depends on the final product produced and the desired properties of the product. Higher amounts of starch will increase the strength of the product. However, it is not possible to increase the amount of starch too much since other problems then may occur. Also, the amount of MFC must be regulated based on the amount of starch and of course also on the end use of the product. Too high amounts of MFC may cause dewatering problems since MFC is a very fine material which easily absorbs water and increased content will make it more difficult to dewater the product.

The paper or paperboard product is preferably a multilayer product comprising at least two layers. It may be preferred that the product comprises at least three layers and that the layer located in the middle of the product comprises furnish comprising starch and microfibrillated cellulose. However, it is also possible that at least one outer layer of the product or even all layers of the product comprises furnish comprising starch and MFC. For some products it might be advantageous that at least one of the outer layers comprises furnish comprising starch and MFC. In this way it is possible to increase the strength and/or the bulk of this layer. Consequently, depending on the end use of the product, it is decided which and how many of the layers that will comprise furnish comprising starch and MFC.

It is not necessary that the entire furnish in a layer of the paper or paperboard product comprises starch and MFC, but it is preferred that the starch and MFC is added to the majority of the furnish of the layer. However, the layer may also comprise other components, such as broke pulp which does not comprise starch and MFC.

The furnish may also contain various amounts of fillers to increase for example runnability and cost-efficiency of the process and the produced substrate. Other commonly used additives used in the production of paper or paperboard can also be added.

The paperboard product is preferably a high quality paperboard product, such a liquid packaging board, graphical board or food service board. The paper product is preferably a high quality paper, such as copy paper of grades A or B, graphical papers, LWC, SC or news paper for high speed printing machines.

The process for producing a paper or paperboard product which process comprises the steps of providing a furnish comprising fibers, adding starch to the furnish, adding microfibrillated cellulose to the furnish and conducting the furnish to a wire in order to form a web. The addition of starch and MFC is preferably done in the machine chest or before the fan pump. It may also be possible to do the addition to the circulation water which later on is added to the furnish. However, all practical points of addition for the starch and MFC can be used as long as there is enough time and mixing of the starch and MFC with the furnish before it is conducted to the wire.

The starch and MFC is added separately. It is preferred to first add starch followed by addition of MFC. It could be possible to mix the starch and MFC before addition to the furnish, however the result is then not as good as when starch and MFC is added separately. It is then possible to mix starch and MFC before addition to the furnish by cooking them, preferably by the use of a jet-cooker. It has been shown that by cooking the mixture of starch and MFC, the MFC shows less tendency to flocculate. Furthermore, by altering the charge and charge density of e.g. starch, different degrees of flocculation can be obtained and it would thus be possible to neutralize the charge of MFC. This might affect dewatering and retention of the fibers and eventual fillers in the product. It may also be possible to add a layer of starch on the MFC and then add this mixture to the furnish, i.e. a multilayering effect can be created.

EXAMPLES Material Used

Bleached chemothermomechanical pulp (BCTMP) at 570 CSF.

Microfibrillated cellulose (MFC) was prepared by refining bleached hardwood sulphite pulp of 4% consistency with edge load of 2 Ws/m to 28 SR. The pulp were thereafter enzymatically treated with Endoglucanase (Novozym 476) with the activity of 0.85 ECU/g. The enzymes were dosed to the pulp and which thereafter was treated at 50° C. for 2 hours, at pH 7. After the enzymatic treatment, the pulp was washed and enzymes were deactivated at 80° C. for 30 min. The pulp was thereafter refined once more to 90-95 SR and the refined pulp was then fluidized (Microfluidizer, Microfuidics corp.) by letting pulp of 3% consistency pass through a 400 μm chamber followed by a 100 μm chamber wherein the MFC used were formed.

Starch used was cationized starch, Raisamyl 70021, Ciba (now BASF). C-PAM used was Percol 292 NS, Ciba (now BASF).

BMA used was Eka NP495, Eka Chemicals.

Example 1

The dried BCTMP were soaked in water over night and then dispersed in hot water. The BCTMP suspension was thereafter diluted to a concentration of 0.3%.

The produced MFC was also diluted to a concentration of 0.3% and dispersed using a kitchen mixer.

A formette sheet former was used to prepare the sheets for testing. The sheets were prepared according to the following procedure; Pulp suspension measured to produce a 150 gsm sheet was added to the stock tank. During agitation, starch if used, and MFC if used, was added. After 30 seconds, 500 g/t C-PAM was added and after another 30 seconds was 300 g/t BMA added to the stock and the sheet forming was thereafter started.

The formed sheet was wet pressed and dried while the shrinkage was constrained. The dried sheet was tested for structural density according to SCAN P 88:01, z-strength according to SCAN P 80:88 and tensile strength index according to ISO 1924-3.

TABLE 1 Results of strength and density tests. Tensile strength Structural Index density z-strength geometrical mean Sample (kg/m3) (kPa) (Nm/g) Reference 570CSF 388 103 27 Addition of 5% MFC 459 218 40 Addition of 5% Starch 396 142 32 Addition of 5% MFC and 436 240 44 2.5% Starch Addition of 5% MFC and 426 256 47 5% Starch

As can be seen from table 1 above it is clear that the addition of the combination of MFC and starch strongly increases the strength, both the z-strength and the tensile index. Furthermore, the densification of the sheets has decreased compared to if only MFC was added.

Example 2

Tests were also performed where MFC and cationic starch either was pre-mixed or added separately to the stock.

When pre-mixed, the MFC and cationic starch was well mixed before addition to the stock. When added separately, cationic starch was first added and well mixed with the stock for 5 minutes followed by addition of MFC.

MFC in an amount of 25 kg/t and cationic starch in an amount of 20 kg/t were added in both samples, both the pre-mixed and the separately added.

After addition of the MFC and cationic starch to the stock, a formette sheet former was used in the same manner as described in Example 1.

As a reference the stock without any MFC and cationic starch was used.

Scott Bond was measured according to TAPPI UM-403.

TABLE 2 Results of strength and density tests. Structural density z-strength Scott Bond Sample (kg/m3) (kPa) (J/m2) Reference 310 170 100 Pre-mixed sample 320 210 125 Separately added sample 320 225 145

As can be seen from Table 2, both z-strength and Scott Bond is increased when cationic starch and MFC is separately added to the stock.

In view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art. However, it should be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention. 

1. A process for producing a paper or paperboard product which process comprises the steps of: providing a furnish comprising fibers, adding starch to the furnish, adding microfibrillated cellulose to the furnish, conducting the furnish to a wire in order to form a web, wherein the starch and microfibrillated cellulose is added separately to the furnish.
 2. The process according to claim 1 wherein 2-15% by weight of starch and 1-15% by weight of microfibrillated cellulose is added.
 3. The process according to claim 1 wherein the web being formed by said furnish comprising starch and microfibrillated cellulose forms a layer of the paper or paperboard product.
 4. The process according to claim 3 wherein the web forms a middle layer of the paper or paperboard product. 